In order to penetrate the egg zona pellucida, mammalian sperm must first undergo a fusion and vesiculation of sperm head membranes called the acrosome reaction. This event normally occurs after several hours in the female tract. However, sperm acrosome reactions can also occur under certain in vitro conditions. A hamster sperm in vitro acrosome reaction-inducing system is being used as a model for the in vivo event. My labratory has found that catecholamines and ovarian follicular fluid serum albumin can stimulate the acrosome reaction. We have also found that the sperm trypsin-like enzyme acrosin and sperm phospholipase may be involved. In addition I propose that sperm ATPases and acrosomal pH may also be involved in the acrosome reaction. The identification of the roles of these endogenous and exogenous factors and the elucidation of the nature of their interaction in the acrosome reaction are the objectives of this work.